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Malaysia Airlines flight MH370 dropped to altitudes as low as 5000 feet, or 1524 metres, using a dangerous flying technique called "terrain masking" to avoid radar in at least three countries, investigators believe.

The plane with 239 people on board also kept to commercial airline routes as it flew for more than seven hours after turning back from its scheduled flight path over the South China Sea, they believe.

As the focus of police investigators has swung back to the pilots, authorities have revealed that whoever spoke to Kuala Lumpur air traffic control minutes after the plane's main communication system was disabled gave no clue that anything was wrong.

"All right, good night," some-one in the cockpit had said calmly, seemingly to mislead ground control that anything was wrong.

Investigators believe that by following commercial routes the plane did not raise the suspicion of people monitoring radar of the countries it overflew.

Terrain masking is used by military pilots for stealth flights.

Experts say flying a Boeing 777 in such a way would be dangerous, putting pressure on the 250 tonnes air frame and possibly causing those on board to be air sick.

The New Straits Times newspaper in Kuala Lumpur is quoting investigators as saying the "person who had control over the aircraft has a solid knowledge of avionics and navigation and left a clean track."

It quotes investigation sources as confirming the plane flew low over Kelantan in peninsular Malaysia.

The number of countries involved in the search for the plane has increased from 14 to 22 after new information revealed the plane's last confirmed position was in one of two corridors, one stretching from the border of Central Asian countries Kazakhstan and Turkmenistan to northern Thailand and a southern corridor stretching from Indonesia to the southern ocean.

Authorities in Kuala Lumpur insist they are giving no greater priority to either corridor and have appealed for countries to deploy more ships and planes to a vast expanse of water in the Indian Ocean west of Australia, where there is no land mass for thousands of square kilometres.

But it is understood investigators say it is more likely the plane is in the northern corridor where there are unstable governments, mountainous terrain and extremist groups.

Malaysia has asked countries to make data on their satellites available to assist the search but some have not because of sensitivities sharing information that involves national security.

Malaysia has also asked for more surveillance aircraft and ships, particularly in the southern corridor closer to Australia.

Malaysia's defence minister Hishammuddin Hussein said the search has swung from mainly shallow seas in the South China Sea to "large tracts of land crossing 11 countries, as well as deep oceans."

Experts are trying to nail down precisely how far the plane could have flown with the fuel on board but sources say the plane would have been close to running out at the time it was recorded in raw satellite data at 8.11am Malaysian time, after departing Kuala Lumpur airport at 12.31am.

Malaysia Airlines has confirmed that no additional fuel than normal was loaded on to the plane in Kuala Lumpur.

It had enough fuel for the almost six hour flight to Kuala Lumpur plus 45 minutes of fuel to spare in case of diversion to another airport.

Investigators are factoring in that plane would have burnt more fuel flying in the denser lower air.

Erratic manoeuvres would also have eaten into the fuel's reserves.

After a week of false leads, misinformation and confusion around one of the world's most baffling aviation mysteries, authorities in Kuala Lumpur have calculated from Immarsat satellite data the plane sent out six "pings," which are also known as electronic handshakes, after its communications equipment was deliberately disabled by some-one on board.

The seventh ping never came, indicating the plane either landed and the engine was shut off or it crashed.

Police have revealed that all of the passengers on board were found in initial checks to not have suspicious background, including two Iranians travelling on stolen passports.

But confirmation the plane veered far off course has intensified police probes into all those on board, as well as airport ground crew who came into contact with the plane before departure.

Malaysia's police chief Khalid Abu Bakar said some foreign intelligence agencies have yet to respond to requests to check whether anyone on board had links to terrorist or extremist groups.

The psychological and personal backgrounds of all on board are also being scrutinised.

Police have searched the homes of both the senior pilot Zaharie Ahmad Shah, 52, and co-pilot Fariq Abdul Hamid, 27, and taken away computers and other items.

They have seized a flight simulator in Zaharie's home and reassembled it at a police complex where it is being forensically examined by experts.